1. Field of the Invention
The present invention is related to a communications device. More specifically, the present invention is related to an IP telephone having an on-chip native device relay, and an on-chip filter termination for a constant impedance filter that is located off-chip.
2. Background Art
In addition to data communications, the Internet can also be used to carry voice telephony. One conventional system that carries voice communications over the Internet utilizes an Internet Protocol (IP), and such telephones are referred to as IP telephones.
The data terminal equipment (DTE) of an IP telephone includes a telephone line that is connected to a computer device through a series-connected relay (i.e. switch). The relay switches an incoming telephone signal to either the computer or to a filter that is connected in parallel with the computer. The filter is connected/disconnected across the computer depending on the state of the IP phone system by closing/opening the associated relay. In a no power or “discovery” mode, the relay is switched so the filter is connected across a physical layer input of the computer. Therefore, the filter receives an incoming signal on the telephone line and reflects low frequency signals back down the telephone line, without the incoming signal reaching the physical layer of the computer. The reflected low frequency signals indicate that a compatible IP phone is available for use. When power is applied to the relay in a “normal operation” mode, the relay is switched so the filter is disconnected from the input of the physical layer of the computer. Therefore, the filter does not effect the incoming signal, and the incoming signal is applied to the physical layer of the computer for further processing.
The continual opening and closing of the relay creates wear and tear of the relay components as the conventional IP phone switches between the discovery and normal modes, eventually causing component failure. It would be more cost-effective to keep the filter connected at all times, thereby eliminating relay replacement. Additionally, the conventional relay is not integrated with the computer or the filter, which increases the manufacturing part count and ultimately the manufacturing cost of an IP Phone.
The conventional IP telephone also includes a signal termination circuit that provides a good input impedance for the incoming signal when the filter is not connected across the computer. Proper signal termination is necessary to provide a good signal match, which aids in proper signal reception during the normal operation mode. The termination circuit is a separate off-chip device, which increases the manufacturing part count and ultimately the manufacturing cost of an IP Phone. It is desirable to integrate the termination circuit in order to reduce the part count during the manufacturing process.
The filter in the conventional IP telephone is a conventional lowpass filter. The conventional lowpass filter has an input impedance that is highly dependent on the frequency of the input signal that is delivered to the filter. Input frequencies that are outside of the filter passband are substantially reflected, which can produce an undesired high return loss. Also, conventional filters are highly sensitive to variations in the filter components and in the variation of components that are connected to the filter.
What is needed is a filter that has a constant impedance for all frequencies, even frequencies that are outside the passband of the filter. Furthermore, the filter should be relatively insensitive to component variation.